Unsaturated aliphatic monocarboxylic acids such as methacrylic acid, and the esters of such acids such as methyl methacrylate, are widely used for the production of corresponding polymers, resins and the like. Typically, a saturated aliphatic monocarboxylic acid, such as propionic acid (PA), can be catalytically reacted with formaldehyde (FA) to produce an alpha, beta-ethylenically unsaturated aliphatic monocarboxylic acid, such as methacrylic acid (MA), and water. The produced alpha, beta-ethylenically unsaturated monocarboxylic acid can be esterified to a polymerizable, alpha, beta-ethylenically unsaturated aliphatic monocarboxylic acid ester, such as methyl methacrylate (MMA), and water.
MMA is a monomer containing a carbon-carbon double bond ##STR1## and a carbonyl group ##STR2## Polymers derived from MMA are sometimes also referred to as acrylic or acrylic-type polymers. The MMA-type polymers have good transparency, weatherability and physical strength properties. Typical end-uses for MMA-derived polymers include acrylic sheet that can be fabricated into signs, advertising displays, lighting fixtures, glazing materials, structural panels and the like, molding resins for automobile tail-light lenses, plumbing fixtures and the like, as well as constituents of a variety of surface coatings, adhesives, inks, floor polishes and the like.
Generally, the condensation reaction to produce an alpha, beta-ethylenically unsaturated aliphatic monocarboxylic acid, such as MA, takes place in the vapor or gas phase and in the presence of a basic or acidic catalyst, a substance which speeds up the rate at which a thermodynamically allowable chemical reaction takes place. In the absence of the catalyst, these reactants require addition of heat energy to overcome an "energy of activation" of the reaction, which can be a barrier to formation of the desired products. Also, in the case where the reactants form a variety of products, a catalyst may tend to increase the rate of formation of one product relative to one or more of the other products. Such a catalyst is said to have increased selectivity to that particular product.
Commercial catalyst suitable for MA production are complex composition of matter comprising a relatively high-area solid support and a catalytically active ingredient on the support. The precise chemical and physical structure of the catalyst and its support or carrier determine the effectiveness of the catalyst per se.
It is generally well-recognized by those skilled in the art that the activity of a catalyst is determined by a variety of catalyst physical properties, such as surface area, pore size, pore-size distribution, surface hydration and oxidation, and the like. Occasionally, the structure or physical state of the support or carrier material per se--e.g. amorphous or crystalline state--affects catalyst performance. Some of these physical properties, such as porosity and surface area, are related. Accordingly, a modification of the method of catalyst preparation from a known procedure so as to desirably alter one property may undesirably alter an other property. Thus, to achieve desired catalytic activity it is often necessary to precisely control catalyst production parameters to produce a catalyst or catalyst support having a number of relatively specific physical properties.
The activity of a catalyst is another important consideration; higher activity manifests itself in a relatively lower temperature for a given conversion. The activity of a catalyst is the relative ease or difficulty of the catalyst to effect chemical conversion of the reactants to desired products, at a given temperature. At a particular temperature, for example, a commercially acceptable percentage of the reactants may be converted to the desired product, with only a relatively minor percentage of the reactants being converted to an undesired by-product or undesired by-products. Typically, an increase in the temperature of the reaction not only tends to increase the rate at which the reactants are converted to the desired product or products, but may also tend to increase the rate at which the undesired by-products are produced as well.
Catalysts which are commonly used for reacting PA with FA to produce MA are alkali metals supported on silica. Typical catalysts of this type are disclosed in U.S. Pat. No. 4,147,718 to Gaenzler et al., U.S. Pat. No. 3,933,888 to Schlaefer, U.S. Pat. No. 3,840,587 to Pearson, U.S. Pat. No. 3,247,248 (see also Canadian Pat. No. 721,773) to Sims et al., and U.S. Pat. No. 3,014,958 to Koch et al.
The teachings of these various references can be readily distinguished from the present invention. In particular, U.S. Pat. No. 4,147,718 to Gaenzler et al. discloses a catalyst composition which necessarily includes Al.sub.2 O.sub.3, TiO.sub.2, or both. Neither U.S. Pat. 3,840,587 to Pearson nor U.S. Pat. No. 3,014,958 to Koch et al. specifically shows activity of a catalyst comprising cesium on a silica support. Also, the Pearson patent specifically teaches away from the present invention in that the Pearson patent is directed to a catalyst having a significantly greater surface area than the catalyst of the present invention. U.S. Pat. No. 3,933,888 to Schlaefer and U.S. Pat. No. 3,247,248 to Sims et al. each disclose a catalyst having a markedly different cesium concentration from that contemplated by the present invention. Moreover, the Schlaefer patent is directed to pyrogenic silica as the catalyst as well as the catalyst support material, and specifically teaches away from using silica gel as the catalyst support material.
Also, these prior-art catalysts, while effecting condensation of PA with FA to MA, unfortunately also generate appreciable amounts of undesirable by-products that have to be separated. Relatively low conversion and/or commercially unacceptable selectivity performance, together with relatively low catalyst useful-life are additional drawbacks of these prior-art catalysts.
Generally, when PA and FA are reacted in the vapor phase and in the presence of a catalyst to produce MA and H.sub.2 O, a number of undesirable by-products are simultaneously produced as well. The more common of these undesirable are 2,5-dimethyl-2-cylopenten-1-one (by-product A), 2,4,4-trimethyl-gamma-butyrolactone (by-product B), 3-pentanone (by-product 3-P), and ethyl isopropenyl ketone (by-product EIK). The presence of these by-products is generally undesirable because current MA-esterification and MMA-polymerization technology requires separation of these by-products either from the MA before it is esterified to MMA, or before the produced MMA is polymerized. It is additionally desirable to remove by-product A from the MA prior to esterification as the presence of this by-product tends to cause an undesirable polymerization of MA and attendant separation problems. Loss of product also may become significant.
A number of the conventional catalysts that are used to produce alpha, beta-ethylenically unsaturated monocarboxylic acids, such as methacrylic acid, are known to undergo a short-term deactivation while on feed. In practice, such short-term catalyst deactivation is overcome by interrupting the feed and then treating the catalyst with an air burn to remove accumulated carbon or other organic material. This procedure is herein referred to as "decoking" of the catalyst. It is desirable to extend the length of time on feed between such de-cokings, not only to optimize productivity of the desired product but also to increase overall efficiency of the process as well. It is also desirable to reduce formation of coke and tar on the catalyst, the formation of which causes loss of catalyst activity and thus decreases the short-term life of the catalyst.
Accordingly, it would be desirable to have a catalyst which provides improved PA conversion and improved selectivity to MA, which decreases undesirable by-product generation, and which enhances useful catalyst life as well. The catalyst of the present invention meets the foregoing desires.